Could you make the plugin customizable, so that the user can enter his/her own API key from GoodReads?

It already is, it just hasn't been documented how.

Not knowing how many people might download this plugin when I published it I would rather not have suddenly had Goodreads suspicious of a mass influx of devkey accounts. If I publish the info here its in the public domain and I would rather hold off on that until it becomes necessary. Lets see how things go and if Goodreads start sending me warnings about volumes then I'll revisit it.